In a typical radio receiver, an antenna receives radio frequency signals and converts them into electrical radio frequency signals. These radio frequency signals are then reduced to a lower frequency signal for amplification in an intermediate frequency (IF) stage. In the IF stage, the signal received from the antenna is amplified and bandpass filtered. Next, the signal is split and input to a quadrature demodulator. The demodulator reduces the frequency of the signal to the base band frequency and prepares the signals for extraction of the useful information. The base band circuitry filters, amplifies and processes the signal extracting the analog or digital data.
A source of distortion in the base band frequency signal occurs when there is a change in the received signal strength. This can occur when a receiver changes from a first frequency to a second frequency, when the source of the received signal changes, or when the receiver changes position relative to the source of the received signal. The relationship between the two signal strengths results in a change of the gain requirements of the variable gain amplifiers (207, 245, 247). Adjustment of the gain of the amplifiers (207, 245, 247) results in a DC component in the base band signal. Adjustment of the gain in the first amplifier (207) may result in a larger DC component than the adjustment of the gain in the other two amplifiers (245,247) in the base band circuitry. Because this amplifier 207 is located in the IF stage of the receiver 107, if the gain is increased, the amount of local oscillator (LO) frequency coupled into this amplifier will increase the DC offset. If there is no LO coupled into this amplifier, than there will be no increase in the DC offset. If the gain on the second amplifier 245 is increased, the DC offset of the BB signal will definitely increase. These changes in DC offset of the first and second amplifiers 207, 245 will result in a DC component significantly larger than an unamplified DC component from the last amplifier (247), because the output from the first and second amplifiers 207,245 are amplified by the last amplifier 247.
The effects of the undesired DC component in the base band signal are illustrated in FIG. 4. FIG. 4A illustrates an ideal representation of an arbitrary signal set represented in signal space by axis 401 projected on to the inphase and quadrature axis 403. FIG. 4B illustrates the result of adding an undesired DC offset 409 to either the I or Q component of the base band signal. The shifts in the I and Q components cause the decision threshold of the I and Q axis 403 to shift, thus, biasing the selection of one symbol over the other, reducing the noise margin for some of the symbols and allowing a reduced margin for error in the presence of uncorrelated noise. The decision thresholds are represented by the I and Q axis 403.
Digital radio receivers have a lower tolerance to DC offset errors than a conventional analog radio receiver. The required change in the gain resulting from a change in frequency of the received signal or change in the source of the received signal is transitory in nature and occurs when the signal being received does not contain any desired data. Thus, the gain of any of the amplifiers (207, 245,247) can be adjusted to provide the proper amplitude of the output signal and the DC offset can be removed as discussed in U.S. Pat. application Ser. No. 07/630,644.
The required change in the gain resulting from a change in the radio receiver's position can occur while the received signal contains desired data. Therefore, the undesired DC offset can not be properly removed, therefore, any significant error will adversely effect the results of the interpolation of the data from the output signal. In order limit the change in the DC component of the output signal, only the gain of the last amplifier should be adjusted while receiving an input signal. The input signal being an RF signal containing desired data and input into the receiver. The output signal defined as the base band signal which is output from the receiver 107.
Therefore, there exists a need for a device to adjust the gain of all the adjustable gain amplifiers after a change in the input signal frequency or change in the source of the input signal and adjust the gain of the last adjustable gain amplifier while receiving an input signal.